One species of photovoltaic cells currently under development employs cadmium sulfide (CdS) and cuprous sulfide (Cu.sub.2 S) which act to form a heterojunction at the interface of the two materials. The CdS may conveniently be provided in a polycrystalline form rather than a single crystal, and the heterojunction formed by converting a portion of the crystalline surfaces to Cu.sub.2 S. The efficiency of the CdS--Cu.sub.2 S photovoltaic cells has traditionally been lower than the efficiency available from photovoltaic cells using the more expensive silicon, and much effort is being expended to improve the efficiency of the CdS--Cu.sub.2 S species of photovoltaic cells.
One developing technique for forming the polycrystalline CdS layer employs a technique for depositing CdS by spraying a chemical solution on a heated substrate. Particular embodiments of such techniques are more particularly described in U.S. Pat. Nos. 3,880,633 and 3,902,920, and in U.S. Pat. Application Ser. No. 631,815 now U.S. Pat. No. 4,086,101 and Ser. No. 670,625 now U.S. Pat. No. 4,095,006. These techniques, briefly, involve heating a glass substrate to substantially a uniform temperature, spraying a first compound to form a layer of electrically conductive tin oxide on the glass, spraying one or more compounds which react on the heated tin oxide surface to form a layer of CdS in the range of four microns thick, annealing the completed films in dry air, forming a layer of Cu.sub.2 S on the surface of the CdS, vacuum depositing electrode materials and then heat treating the completed cell. Formation of the CdS layer involves the use of various cadmium salts, including cadmium chloride. Residual chlorides remain in the CdS crystal structure after the CdS film has been formed to the desired thickness. The residual chlorides are desirable at this stage to enhance the growth of the CdS crystallites in subsequent heat treating operations. However, it has been found that the presence of residual chlorides in the CdS film appear to accelerate the degradation of the performance of the photovoltaic cells throughout the cell lifetime. In addition, the presence of chlorides adjacent the surface of the CdS crystallites appears to effect heterojunction formation and reduce the available open circuit voltage.
A conventional metal used to form the positive electrodes on photovoltaic cells has been copper. Copper is abundant and easily applied but produces some unfavorable results. First, copper does not always make good ohmic contact with the Cu.sub.2 S layer, and a reduced cell output voltage results. Second, copper tends to diffuse throughout the photovoltaic cell over the operating lifetime of the cell. This diffusion is accelerated by high temperatures, such as the final heat treatment of the completed cell, or such as might occur in certain operating locales. It is, therefore, desirable to investigate alternate electrode materials which do not act to degrade photovoltaic cell performance.
The disadvantages of the prior art, however, are overcome by the present invention and improved methods are provided for fabricating a photovoltaic cell having chromium positive electrodes and producing an improved open circuit output voltage.